1. Field of the Invention
This invention relates to a process for producing transformer oil feedstocks. More particularly, this invention relates to a double solvent extraction-hydrotreating process for producing transformer oil feedstocks from raw, untreated light distillate fractions obtained from paraffinic crude oils. The transformer oil feedstock may then be dewaxed to produce a low pour point transformer oil.
2. Description of the Prior Art
Transformer oils are high stability electrical insulating oils used in transformers and in other electrical equipment such as circuit breakers. In transformers the oil provides two major functions. The first is as an insulator and the second is as a heat transfer medium to carry heat from the coils to the cooling surfaces of the transformer. These oils must be low in corrosive agents such as acid, alkali and sulfur and resistant to oxidation and sludge formation. In addition to possessing relatively low viscosity, high electric strength and a relatively high flash point, these oils are further characterized in that they must have a relatively low pour point. This is particularly necessary when the oils are used in transformers in colder climates.
These transformer oil stocks are generally produced from relatively wax-free naphthenic crude oils which are not native to many parts of the world and consequently command premium prices and involve high transportation costs. Further, the production of naphthenic crude oils is rapidly diminishing due to government takeover of heretofore privately owned wells and drying up of others. At the same time, demand for these oils continues to increase as industry and modern civilization continue its growth. Therefore, there is an ever increasing need to produce these oils from paraffinic crudes. Extremely stable insulating oils produced either totally or partially from paraffinic crudes by conventional dewaxing techniques are used in certain applications where moderate climatic conditions do not demand oil with especially low cloud or pour points. However, when exceptionally low pour points are required, deep dewaxing of paraffinic distillates at temperatures below -40.degree. F. cannot compete economically with the manufacture of these oils from naphthenic crudes.
Fortunately, however, low pour point transformer oils can be obtained from paraffinic crudes by using, as a transformer oil feed, a narrow cut paraffinic fraction having a 5 to 95LV% (liquid volume) boiling range between about 595.degree. to 750.degree. F. Certain well fractionated, narrow cut, low viscosity distillates taken as a heart cut distillate exhibit a pour-filter inversion when ketone dewaxed, in which the pour point of the dewaxed oil can be as low as 30.degree. to 40.degree. F. below the wax filtration temperature. In contrast, conventional (broad cut) distillates when ketone dewaxed generally exhibit a pour point about 5.degree. F. above the wax filtration temperature. This is disclosed in U.S. Pat. Nos. 2,906,688, 3,627,673 and in pending application, U.S. Ser. No. 599,690 (filed July 28, 1975) now U.S. Pat. No. 4,018,666. For the purposes of the instant invention these distillates shall be referred to as transformer oil feedstocks and have a viscosity ranging from about 40 to 70 SUS at 100.degree. F. with a 5 to 95LV% boiling range between about 595.degree. to 750.degree. F.